1. Field of the Invention
This invention relates to an organic electroluminescent device and to a method for manufacturing the organic electroluminescent device.
2. Description of the Related Art
In order to enhance the light extraction efficiency in an organic electroluminescent device (organic EL device), there has been conventionally proposed a technique of forming a diffraction grating inside the EL device. The guided light inside the EL device is diffracted by the diffraction grating before it is retrieved from the EL device. As a result, it is possible to enhance the light extraction efficiency as compared with the EL device which is not provided with the diffraction grating. However, in the case of organic EL device where a diffraction grating having the same periodic axis throughout the entire region of emission surface is provided, non-uniformity in luminance generates in the direction of emission surface, so that such an organic EL device is not suited for use in a display device where uniformity of luminance in the emission surface is required.
The aforementioned diffraction grating is usually formed inside the organic EL device by a process comprising a photolithography step, etc., which is generally employed in a fine working of semiconductor device. In the formation of a pattern of fine recessed and projected portions, especially a very fine pattern where the pitch thereof is 1 μm or so, it is required to employ an expensive apparatus and a complicated process, thus raising a problem that the manufacturing cost of organic EL device would become very high.
There has been also proposed a technique of enhancing the light extraction efficiency through the control of the guided light in the organic EL device, wherein a light diffusion layer is provided in a transparent matrix and particles having a different refractive index from that of the matrix are dispersed in the light diffusion layer. The light emitted at the emission portion scatters by the particles arranged at random in the matrix, thereby suppressing the guided light in the EL device. In this case, it is possible to prevent the luminance from becoming non-uniform in the emission surface. However, due to the particles arranged at random in the matrix, the light emitted in the direction perpendicular to the substrate at the emission layer also scatters in various directions raising a problem that the front face luminance is prominently degraded.
Therefore, there is persistent demand to enhance the light extraction efficiency of the organic EL device without deteriorating the uniformity of luminance in the emission surface.